THz radiation (T-rays) occupies a large portion of the electromagnetic spectrum between the infrared and microwave bands, namely the frequency interval from 0.1 to 10 THz, and is a developing frontier in imaging science and technology. In contrast to the relatively well-developed techniques for medical imaging at microwave and optical frequencies, however, there has been only limited basic research, new initiatives and advanced technology developments in the THz band. THz waves have been increasingly used to characterize the electronic, vibrational and compositional properties of solid, liquid and gas phase materials.
Unlike X-rays, T-rays have low-photon energies (4 meV @ 1 THz), low average power (nW to μW) and do not subject biological tissue to harmful radiation. T-rays can be focused to give sharper pictures. In addition, T-rays give spectroscopic information about the chemical composition as well as the shape and location of the targets they are imaging. This combination of information of the physical and the biochemical nature of the imaged tissue may be of particular value for clear and early diagnosis and detection of diseases such as cancer, allowing for a choice of treatment options.
Unlike common optical spectroscopes, which only measure the intensity of light at specific frequencies, THz time-domain spectroscopic techniques directly measure the THz wave's temporal electric field. Fourier transformation of this time-domain data gives the amplitude and phase of the THz wave pulse, therefore providing the real and imaginary parts of the dielectric constant without the use of the Kramers-Kronig relations. This allows precise measurements of the refractive index and absorption coefficient of samples that interact with the THz waves. Many rotational and vibrational spectra of various liquid and gas molecules lie within the THz frequency band, and their unique resonance lines in the THz wave spectrum allow us to identify their molecular structures. Raman spectroscopy directly uses the frequency domain to fingerprint the lattice vibrations. Similarly, THz wave spectroscopy describes molecular rotational and vibrational spectra from 10 GHz to 10 THz using the real and imaginary parts of the dielectric function that are obtained by measuring the THz wave in the time-domain. Current optical or microwave techniques cannot achieve this functionality.
Due to the diffraction-limit, the standard imaging resolution for 1 THz has historically not been much smaller than 300 μm. Near-field imaging techniques are known that can greatly improve the spatial resolution of a THz wave sensing and imaging system. Collection mode near-field imaging has been demonstrated to improve spatial resolution as low as a 7 μm imaging resolution with 0.5 THz pulses. A limitation of such a system, however, is the extremely low throughput of the THz wave past the emitter aperture, because the throughput THz wave field is inversely proportional to the third power of the aperture size of the emitter aperture. Therefore, pre-existing THz wave generation and detection technologies are inadequate for obtaining sub-micron spatial resolution.
A newly developed dynamic-aperture method with the introduction of a third gating beam can image objects with a sub-wavelength resolution (λ/100), but the drawback of this method is the difficulty in coating a gating material on the surface of biomedical samples such as cells and tissues.
Thus, there is a need in the art for a T-ray imaging technique and system that can provide imaging with submicron resolution using THz radiation.